# ET.ChapmanAustralian

##### Chapman Formulation

Implementing the Chapman formulation for estimating potential evapotranspiration.

##### Usage

```
# S3 method for ChapmanAustralian
ET(data, constants, ts="daily", PenPan= T, solar="sunshine hours", alpha=0.23, …)
```

##### Arguments

- data
A list of data which contains the following items (climate variables) required by Chapman formulation:

*Tmax*,*Tmin*,*RHmax*,*RHmin*,*Rs*or*n*or*Cd*,*u2*or*uz*- constants
A list named

`constants`

consists of constants required for the calculation of Chapman formulation which must contain the following items:*Elev*- ground elevation above mean sea level in m,*lambda*- latent heat of vaporisation = 2.45 MJ.kg^-1,*lat_rad*- latitude in radians,*Gsc*- solar constant = 0.0820 MJ.m^-2.min^-1,*z*- height of wind instrument in m,*sigma*- Stefan-Boltzmann constant = 4.903*10^-9 MJ.K^-4.m^-2.day^-1,*lat*- latitude in degrees,*alphaA*- albedo for Class-A pan,*ap*- a constant in PenPan = 2.4.The following constants are also required when argument

`solar`

has value of`sunshine hours`

:*as*- fraction of extraterrestrial radiation reaching earth on sunless days,*bs*- difference between fracion of extraterrestrial radiation reaching full-sun days and that on sunless days.- ts
Must be either

`daily`

,`monthly`

or`annual`

, which indicates the disired time step that the output ET estimates should be on. Default is`daily`

.- PenPan
Must be

`T`

or`F`

, indicating if the PenPan formulation is used for estimating Class-A pan evaporation required in Chapman formulation. If`T`

PenPan will be used and if`F`

the actual data of Class-A pan evaporation will be used. Default is`T`

for using the PenPan formulation.- solar
Must be either

`data`

,`sunshine hours`

,`cloud`

or`monthly precipitation`

:`data`

indicates that solar radiation data is to be used directly for calculating evapotranspiration;`sunshine hours`

indicates that solar radiation is to be calculated using the real data of sunshine hours;`cloud`

sunshine hours is to be estimated from cloud data;`monthly precipitation`

indicates that solar radiation is to be calculated directly from monthly precipitation. Default is`sunshine hours`

.- alpha
Any numeric value between 0 and 1 (dimensionless), albedo of the evaporative surface incident radiation that is reflected back at the surface. Default is 0.23 for surface covered with short reference crop.

- …
Dummy for generic function, no need to define.

##### Details

The alternative calculation options can be selected through arguments `PenPan`

and `solar`

, please see `Arguments`

for details.

##### Value

The function prints a calculation summary to the screen containing the following elements:
- ET model name and ET quantity estimated, and the value of pan coefficient (only for when potential ET is estimated)
- Evaporative surface with values of albedo
- Option for calculating solar radiation (i.e. the value of argument `solar`

)
- If the PenPan formulation is used for estimating Class-A pan evaporation required in Chapman formulation (i.e. the value of argument `PenPan`

)
- Time step of the output ET estimates (i.e. the value of argument `ts`

)
- Units of the output ET estimates
- Time duration of the ET estimation
- Number of ET estimates obtained in the entire time-series
- Basic statistics of the estimated ET time-series including *mean*, *max* and *min* values.

The function also generates a list containing the following components, which is saved into a `csv`

file named as *ET_ChapmanAustralian.csv* in the working directory:

Daily aggregated estimations of Chapman potential evapotranspiration.

Monthly aggregated estimations of Chapman potential evapotranspiration.

Annually aggregated estimations of Chapman equivalent Penmen-Monteith evapotranspiration.

Monthly averaged estimations of daily Chapman potential evapotranspiration.

Annually averaged estimations of daily Chapman potential evapotranspiration.

Name of the formulation used which equals to `Chapman`

.

Type of the estimation obtained which is `Potential Evapotranspiration`

.

A message to inform the users about how solar radiation has been calculated by using which data.

A message to inform the users about if the Class-A pan evaporation is from actual data or from PenPan estimation.

##### References

McMahon, T., Peel, M., Lowe, L., Srikanthan, R. & McVicar, T. 2012. *Estimating actual, potential, reference crop and pan evaporation using standard meteorological data: a pragmatic synthesis*. Hydrology and Earth System Sciences Discussions, 9, 11829-11910.

Chapman, T. 2001, *Estimation of evaporation in rainfall-runoff models*, in F. Ghassemi, D. Post, M. SivapalanR. Vertessy (eds), MODSIM2001: Integrating models for Natural Resources Management across Disciplines, Issues and Scales, MSSANZ, vol. 1, pp. 293-298.

##### See Also

##### Examples

```
# NOT RUN {
# Use processed existing data set and constants from kent Town, Adelaide
data("processeddata")
data("constants")
# Call ET.ChapmanAustralian under the generic function ET
results <- ET.ChapmanAustralian(data, constants, ts="daily", PenPan= TRUE,
solar="sunshine hours", alpha=0.23)
# }
```

*Documentation reproduced from package Evapotranspiration, version 1.10, License: GPL (>= 2)*